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Original Papers: Franklin, K. A., S. J. Davis, W. M. Stoddart, R. D. Vierstra and G. C. Whitelam. 2003. Mutant analyses define multiple roles for phytochrome C in Arabidopsis photomorphogenesis. The Plant Cell 15: 1981–1989.

Qin, M., R. Kuhn, S. Moran and P. H. Quail. 1997. Overexpressed phytochrome C has similar photosensory specificity to phytochrome B but a distinctive capacity to enhance primary leaf expansion. The Plant Journal 12: 1163–1172.

Sharrock, R. A. and T. Clack. 2002. Patterns of expression and normalized levels of the five Arabidopsis phytochromes. Plant Physiology 130: 442–456.

Phytochromes sense red (R) and far-red (FR) light and control many processes in plant growth and development, including chlorophyll synthesis, seed germination, and leaf expansion. The phytochrome photoreceptor has two parts: a chromophore that receives light and a protein involved in signal transduction for the response (see Figure 36.14). The model plant Arabidopsis has five distinct genes (phyA, phyB, phyC, phyD, and phyE) that control the formation of five types of phytochrome. These genes code for five slightly different proteins, the differential expression of which in different tissues at different times may underlie the varying responses of plants.

Elucidating the roles of the phy genes has involved both molecular and genetic approaches. In one molecular approach, a gene is coupled to a very active promoter and expressed at higher than normal levels (overexpressed) in developing Arabidopsis plants. For example, phyA, phyB, phyC, and phyD were separately overexpressed in Arabidopsis, and 18-day-old seedlings were examined with regard to leaf area. The table shows the results.

To complement the molecular approach, a genetic approach—a genetic screen for plants expressing altered leaf size phenotypes—was used to isolate mutants deficient in phyC (see Figure 36.2). All of the phyC mutants found using the genetic screen turned out to also be genetically deficient in phyD. This result prompted an investigation of plants deficient in different combinations of phy genes. These plants were analyzed with regard to leaf length (which increases due to elongation of the petiole that connects leaf to stem) and leaf area. The results are shown in Figures A and B.

Questions

Question 1

Acting alone, phyC enhances leaf area, whereas phyA, phyB, and phyD do not.

Question 2

Arabidopsis seeds would be treated with a mutagen and planted. Those plants with reduced leaf size would be isolated and grown to examine for phyC mutations.

Question 3

The leaf area phenotypes produced by these three conditions are equal. All three groups have leaf areas about one-third smaller than that of the WT group. This suggests that (1) the phyA and phyC genes have the same effect on controlling leaf area (these results show the effect of the mutant, or nonfunctional gene, so the functional gene should increase leaf area by one-third); (2) either gene will work with equal effect in the absence of the other (shown by phyAD and phyCD); and (3) having both genes dysfunctional does not increase the effect (shown by phyACD).

Question 4

A deficiency in phyB causes a very long leaf (a long petiole) and a very small leaf area (small leaf). This suggests that, if this gene were functional, it would control leaf growth by greatly inhibiting petiole length and greatly increasing leaf area. This would produce a leaf with a large surface area for photosynthesis, which could be highly adaptive. PhyA and phyC have the same effect on leaf area as phyB, but to a lesser extent. PhyA and phyC also inhibit petiole length, with phyA having the greater effect and phyC adding slightly to this effect. It is likely that, in plants in nature, the control of these two leaf features results from the ratio of the various phytochromes present.